X-ray examination apparatus including a dosimeter

ABSTRACT

The invention relates to an arrangement which includes a radiation source ( 1 ) and a radiation sensor device ( 3 ) for forming image signals (Bn), the radiation sensor device ( 3 ) being associated with a read-out circuit arrangement ( 4 ) for the amplification/processing of image signals (Bn) read out, there also being provided at least one dosimeter ( 42 ) which is arranged to measure a radiation dose. The invention also relates to an X-ray examination apparatus and to a method for the processing of X-ray images. In order to realize an arrangement and a method whereby a high image quality can be achieved for the acquired images, it is proposed to apply a dose signal (Dn), produced by at least one dosimeter ( 42 ), to at least the read-out circuit arrangement ( 4 ).

The invention relates to an arrangement which includes a radiationsource and a radiation sensor device for forming image signals, theradiation sensor device being associated with a read-out circuitarrangement for the amplification/processing of image signals read out,there also being provided at least one dosimeter which is arranged tomeasure a radiation dose. The invention also relates to an X-rayexamination apparatus which includes an X-ray tube, an X-ray detector, aread-out circuit arrangement and an image processing device. Theinvention also relates to a method of processing X-ray images, in whichmethod X-rays emitted by an X-ray tube are conducted to an X-raydetector which is arranged to generate image signals, comprises sensorsand is associated with at least one dosimeter, said dosimeter generatingat least one dose signal which is indicative of the intensity of X-rays.

The invention can be used in the field of image pick-up devices. Theradiation sensor device may then be a light-sensitive sensor device aswell as a sensor device which is sensitive to X-rays. Light-sensitivesensor devices are used, for example, in the photographic technique.Sensor devices which are sensitive to X-rays are used in particular inthe medical field but also in the field of materials analysis or in thefield of security. The sensor devices may also be sensitive to aplurality of types of radiation of different wavelength.

Because only comparatively small radiation doses are used for medicalexaminations in order to ninimize the radiation dose whereto the patientto be examined is exposed, the image signals formed from the X-rays arevery susceptible to interference and generally have only a very lowlevel, thus giving rise to sensitivity problems in the sensor devices.The foregoing necessitates complex post-processing of the image signals,so that they can also be further processed when they have a very lowlevel. An increase of the radiation dose so as to generate a strongerimage signal is thus avoided.

Generally speaking, radiation is emitted by a radiation source in sucharrangements. The radiation source may then be constructed as a lightradiation source or as an X-ray tube. The radiation sensor deviceconverts the incident radiation, for example, into charge carriers, orinto radiation of a different wavelength, which is detected by sensorsand wherefrom, for example, electrical image signals are formed. Adifferent radiation dose is then incident on the individual areas orpoints of the sensor surface. In the medical field X-rays are attenuatedto a varying extent due to the varying absorption properties of thebones and the tissue of the patient to be examined, thus giving rise todifferences between the image signals which ultimately result in thecommonly known X-ray images. The radiation sensor device is succeeded bya read-out circuit arrangement which performs, for example,amplification and conversion of the analog image signals into digitalimage signals. The image signals are individually applied to theread-out circuit arrangement, the amplification and conversion yieldingan overall image signal which is displayed/output on, for example, amonitor or another output medium via an image processing device.

EP 0440282 discloses an embodiment of an arrangement with a read-outcircuit which is conceived in particular for X-rays. A furtherarrangement with a dosimeter is disclosed in EP 00308217.9.

The number of image signals to be transferred from the radiation sensordevice to the read-out circuit arrangement is usually very large, sothat a large amount of time is required for this purpose. Such radiationsensor devices are also used inter alia to form moving images, the imagerepetition frequency required for high quality imaging thennecessitating a very high data throughput within a very short period oftime. Limiting the data throughput leads to a limitation of the displayquality.

Therefore, it is an object of the invention to provide an arrangementand a method whereby a high display quality can be achieved for imagesto be acquired.

This object is achieved by means of an arrangement which includes aradiation source and a radiation sensor device for generating imagesignals, the radiation sensor device being associated with a read-outcircuit arrangement for the amplification/processing of image signalsread out, there also being provided at least one dosimeter which isarranged to measure a radiation dose, and in which at least one dosesignal generated by the dosimeter can be applied at least to theread-out circuit arrangement.

The invention is based on the idea that a radiation dose signal intendedto control the radiation capacity of the radiation source can beadvantageously used in accordance with the invention for influencing theprocessing stages of a radiation device.

Depending on the relevant type, dosimeters are integrated in the sensordevice or are arranged, for example, in the form of an ionizationchamber, in front of the sensor device. They generate a dose signalwhich is proportional to the amount of radiation occurring.

Until now this dose signal was used exclusively for the control of theradiation source. In accordance with the invention the informationcontained in the dose signal is also used to influence the downstreamcomponents in that at least one dose signal is applied to the read-outcircuit arrangement.

To this end, in an advantageous embodiment of the invention the sensordevice consists of a plurality of single sensors. The larger the numberof sensors in relation to the surface area of the sensor device, thehigher the resolution of the image formed will be. For simpleapplications it already suffices to generate only one dose signal forthe entire sensor device. In that case only a single dosimeter isprovided. In the case of high resolution sensor devices, a dosimeter maybe associated with each individual sensor in an extreme case. Dependingon the relevant application, it is advisable to form a mean dose signalfor a group of sensors. The formation of one or more mean dose signalscan take place either already in the sensor device or in a unitsucceeding the sensor device.

Parameters of the read-out circuit arrangement can be changed in anadvantageous embodiment of the invention. The dose signal is generatedsimultaneously with the image signals upon exposure of the sensor deviceto be incident rays. The dose signal is then applied to the read-outcircuit arrangement earlier or at least no later than the image signals.The read-out circuit arrangement comprises, for example, amplifiers foramplifying the image signals which are usually very weak and possiblyalso noisy. Furthermore, in order to convert the analog image signalsinto digital image signals there are also provided A/D converters whichcombine the individual image signals so as to form an overall imagesignal which is then suitable for further processing. Depending on thedose signal or the plurality of dose signals, parameters of the imagesignals, for example, the necessary amplification of the image signals,can be changed. To this end, for example, a gain factor for the imagesignals is associated with the individually measured dose signals in alook-up table which is stored in a memory. From the amount of radiationincident on the corresponding sensor, and the dose signal generatedtherefrom, there is thus determined an individual gain factor wherebythe image signal read from this sensor is amplified.

In an advantageous embodiment of the invention a parameter, for example,an offset or gain factor, of the image signals is changed to the sameextent for all image signals of the sensor device, that is, independence on the dose signal. The influencing of other parameters, forexample, the current/voltage supply for the read-out circuitarrangement, is also possible. The type of filtering of all or onlyindividual image signals can also be influenced.

The changing of the parameters in dependence on the dose signal or thedose signals yields an enhanced overall image quality and thearrangement can then also be used for a larger dynamic range.

In an advantageous embodiment of the invention the image signals areapplied to the read-out circuit arrangement in dependence on whether thedose signal exceeds or does not reach a threshold value, for example, inorder to prevent amplification of image signals from overexposed sensorareas. Because an exposed or irradiated object usually does not coverthe entire surface area of the sensor device, individual areas will bestruck by direct radiation Such direct radiation image signals, however,usually are not of interest. Moreover, the usually strong radiation insuch direct radiation areas produces an image signal which is verystrong in comparison with the other image signals of the irradiatedobject and is applied to the read-out circuit arrangement. Thishigh-level image signal usually leads to saturation in thecorrespondingly associated amplifiers. On the basis of the likewise highdose signal from such directly irradiated areas the image signals fromthese areas can be switched off, or excluded from amplification, bycomparison with a threshold value so that on the one hand the imagesignals of the overexposed areas are not applied to the read-out circuitarrangement and on the other hand the amplifiers are not driven into thesaturation range either. Furthermore, an examination zone can be definedon the basis of selectable threshold values.

The radiation source in a preferred embodiment of the invention isformed by an X-ray tube while the radiation sensor device is formed byan X-ray detector.

The invention can be carried out particularly advantageously in X-raydetectors which consist of a plurality of sensors which are sensitive toX-rays. The sensors are then arranged, for example, in rows and columnsin the form of a matrix. The dosimeter measures the radiation doseoccurring during the irradiation. The dose signal is applied to theread-out circuit arrangement before the image signals. It is thenadvantageous to perform the dose measurement for groups of sensors inthe X-ray detector, because the differences in radiation dose betweenneighboring sensors are negligibly small. Preferably there are formedso-called superpixels which consist of a group of individual sensors. Inthis manner the X-ray detector supplies two signals per unit of time foreach individual sensor. A first signal is a mean dose signal which maybe the same for a plurality of sensors. The further signal is an imagesignal which is read out from each individual sensor and has been formedfrom the incident amount to radiation. These two signals are applied tothe read-out circuit arrangement in accordance with the invention.

In a further embodiment of the invention the mean dose signal of asuperpixel is subtracted from the image signals associated with thissuperpixel in the read-out circuit arrangement, so that smallerdifference signals are formed from the image signals. Because the meandose signal offers an indication as regards the value range in which theincident radiation is situated, the formation of the difference signalensures that only the deviation relative to said mean signal is furtherprocessed. A calculation unit may be provided for this purpose.Subsequently, the mean dose signal and the relevant image signals areconverted into digital signals by A/D converters included in theread-out circuit arrangement The amount of image signals to be processedis thus reduced.

In an X-ray sensor device consisting of 1024×1024 individual sensors orpixels with a superpixel for radiation dose measurement which consistsof each time 64×64 sensors, for example, 8 bits can be used forconverting the mean dose signal and 8 bits for converting the differencesignal. Thus, for each image or frame 1024×1024×8 bits and additionally16××8 bits for the mean dose signals would have to be transferred, sooverall only approximately 8 Mbit/frame in the case of a resolution of16 bits. Without the arrangement in accordance with the invention,however, 16 Mbit/frame would have to be transferred for a resolution of16 bits. The range of values of the digitization for the mean dosesignal and that for the difference signals preferably overlap a few bitsso as to cover a larger dynamic range, said overlap not being taken intoaccount in the above numerical example.

In a further embodiment of the invention the mean dose signal of theX-ray detector is used to adapt the conversion range of the A/Dconverters. This enables the same signal resolution for the same signalranges and increases the dynamic range of the X-ray detector. In methodswhich are known from the state of the art the dynamic range of the X-raydetector is usually increased only while accepting non-lineardisturbances in the image signals, for example, by utilizing logarithmicamplifiers.

In a further embodiment of the invention the dose signal is used toadjust the gain factor of the amplifiers for amplifying the imagesignals read out from the X-ray sensor device in the read-out circuitarrangement. The dynamic range of the X-ray sensor device is thusincreased, without it being necessary to form a difference signal fromthe mean dose signal and the relevant image signals. Moreover, using thedose signal from a group of sensors, for example, a column of sensors,it is advantageously possible to adjust only the gain factor of therelevant column to be read out in the X-ray detector.

The sensor arrangement includes large-area electronic circuitry which isadvantageously realized by means of a thin-film technique. The inventioncan also be used in sensor devices utilizing electronic circuitry fromcrystalline silicon, for example, realized in CMOS technology. In thecase of X-ray detectors the conversion of X-ray quanta can be performedby means of scintillators or directly converting materials.

The object is also achieved by means of an X-ray examination apparatuswhich includes an X-ray tube, an X-ray detector for generating imagesignals, a read-out circuit arrangement which is associated with theX-ray detector in order to amplify/process image signals read out, atleast one dosimeter which is arranged to measure a radiation dose, andan image processing device, in which apparatus a dose signal generatedby the dosimeter can be applied to the read-out circuit arrangementand/or the image processing device. The additional information of thedose signal is then also applied to the image processing unit in orderto influence the overall image signal during the further processing. Forexample, the dose signals may already produce a coarsely rasteredrepresentation of the overall image. This represents low-pass filteredimage information which can be advantageously used directly in the imageprocessing, for example, for the formation of a “multi-resolution”decomposition of the image information.

Time can be saved by applying the dose signal to the read-out circuitarrangement in accordance with the invention, that is, notably byreducing the redundancy in the image signals to be transferredFurthermore, the A/D converters may be proportioned so as to be smaller,thus enabling faster conversion.

The object is also achieved by means of a method for X-ray imageprocessing in which X-rays emitted by an X-ray tube are conducted to anX-ray detector which is arranged to generate image signals, comprisessensors and is associated with at least one dosimeter, the dosimetergenerating at least one dose signal which is indicative of the intensityof X-rays, said dose signal being applied to a read-out circuitarrangement no later than simultaneously with the image signals in orderto influence individual parameters of the image signals of theindividual sensors and/or to influence the totality of image signals.

The invention will be described in detail hereinafter with reference tothe drawings. Therein:

FIG. 1 shows an X-ray examination apparatus,

FIG. 2 is a diagrammatic representation of an arrangement in accordancewith the invention,

FIG. 3 is a time diagram of the X-ray exposure, the dose signal, and theimage signal,

FIG. 4 shows a detail of a sensor with a dosimeter,

FIG. 5 is a representation of an overexposed area and the region ofinterest (ROI), and

FIG. 6 shows an embodiment of a sensor element.

FIG. 1 shows an X-ray examination apparatus in which an X-ray tube 1emits X-rays which are first incident on a patient 2 or an object to beexamined and subsequently on an X-ray detector 3. The X-ray detector 3comprises a layer (not shown) in which the X-rays are converted intovisible light radiation. The light radiation is detected bylight-sensitive sensors in which it is converted into electric chargecarriers. According to another possibility the incident X-rays areconverted directly into electric charge carriers. The electric chargecarriers are applied as image signals to a read-out circuit arrangement4 in which they are amplified and converted into digital signals. Fromthe numerous individual image signals from the numerous individualsensors of the X-ray detector the read-out circuit arrangement 4generates a common image signal which is applied to an image processingunit 5 in which the image is further processed. The image produced bythe image processing unit can be output by an output unit, for example,a monitor 6.

FIG. 2 shows an arrangement in accordance with the invention. The X-raydetector 3 comprises a plurality of single sensors (not shown), theX-ray detector 3 also being associated with a dosimeter (not showneither). The single sensors convert the X-rays into electric chargecarriers which are applied to the read-out circuit arrangement 4 asimage signals B_(n). The dosimeter generates a dose signal D_(n) whichis proportional to the incident X-ray dose and, in accordance with theinvention, is also applied to the readout circuit arrangement 4. In FIG.2 a plurality of dose signals D_(n) is generated, because a plurality ofdosimeters is arranged in the X-ray detector 3 in the presentembodiment. The detected dose signals D_(n) are applied to a look-uptable 18 in which a gain factor is associated with each dose value of adose signal D_(n). This gain factor is applied to the correspondingamplifier in the read-out circuit arrangement 4 which amplifies theimage signal B_(n) which originates from the region or the pixel onwhich the corresponding dose D_(n) was measured.

FIG. 3 shows a time diagram The signal waveform A shows the time windowA_(XRay) in which reading does not take place. A_(Readout) denotes thetime window in which the image signals B_(n) are read out. The signalwaveform B shows the time window B_(XRay on) in which exposure by meansof X-rays takes place. In the remaining time B_(XRay off) the X-ray tubedoes not emit X-rays. The signal waveform C shows the rise of a dosesignal D_(n) as a function of the time of the X-ray exposure within thetime window A_(XRay). The dose signal is then available already duringthe exposure and prior to the reading out. The signal waveform D showsthe instant at which the image signal B_(n) of a pixel is read outwithin the time window A_(Readout).

FIG. 4 shows a detail of an X-ray detector 3. The X-ray detectorcomprises a plurality of single sensors or pixels. For the sake ofclarity, only one sensor 39 is explicitly indicated. The detail showncomprises 16 sensors in total. Each sensor includes a dosimeter 42 and aswitching unit 43; a sensor also includes a light-sensitive or X-raysensitive element 67 and a charge carrier collector element 65 (notshown). Via control leads 46, the row control unit 40 controls thesensors of each time one row, so that the charge carriers generated canbe applied as image signals B₁ to B₄, via the read-out leads 45, to theamplifiers 44 and subsequently to the read-out circuit arrangement 4.The X-ray dose in each sensor is measured over the exposure time bymeans of the dosimeters 42. In the present embodiment the dose signalsfrom each time four neighboring sensors are combined and averaged. Themean dose signals D_(n) of the each time four sensors are applied, viathe read-out leads 47 to 50, to the amplifiers 44. The dose signals D₁to D₄ thus generated are applied to the read-out circuit arrangement 4,said amplifiers 44 forming part of the read-out circuit arrangement FIG.5 shows a patient 2 whose leg has been X-rayed. The X-ray image 53formed contains directly irradiated regions 51 and a region 52 in whichthe X-rays have been attenuated by the patient. This region 52 is alsoreferred to as the Region Of Interest (ROI). Sensors which are situatedin the region 51 have a very high dose signal as opposed to those whichare situated in the region 52. Using a threshold value, the amplifiersin the read-out circuit arrangement can be switched off for all imagesignals for which the dose signal exceeds the threshold value, oramplification of the corresponding image signals can be inhibited. As aresult, only the image regions which are actually of interest arefurther amplified and displayed, without falsifying overexposed regionsbeing displayed.

FIG. 6 shows a single sensor 39. This sensor includes a light-sensitiveor X-ray sensitive element 67, a capacitor 65 which serves as a chargecarrier collecting element, and a switching transistor 63. A capacitor62 is provided as a dosimeter. The control lead controls the switchingtransistor 63 in such a manner that the charge carriers collected in thecapacitor 65 can be read out via the read-out lead 45. The dose signalis present on the junction 66 wherefrom it can be read out directly viaread-out leads (not shown). However, as is shown in FIG. 4, it can alsobe combined with the dose signals from other sensors so as to beaveraged.

1. An apparatus including: a radiation source and a radiation sensordevice for forming image signals, the radiation sensor device beingassociated with a read-out circuit arrangement for theamplification/processing of image signals read out, there also beingprovided at least one dosimeter which is arranged to measure a radiationdose, and in which at least one dose signal generated by the dosimetercan be applied at least to the read-out circuit arrangement.
 2. Theapparatus of claim 1, wherein the dosimeter is integrated in theradiation sensor device.
 3. The apparatus of claim 2, wherein theradiation sensor device consists of a plurality of single sensors. 4.The apparatus of claim 3, wherein at least one dosimeter is associatedwith at least one sensor of the radiation sensor device.
 5. Theapparatus of claim 4, wherein at least one dosimeter generates each timea dose signal from groups of sensors and that group-wise influencing ofthe image signals formed by the group of sensors takes place in theread-out circuit arrangement.
 6. The apparatus of claim 1, wherein thedose signal can be applied to the read-out circuit arrangement no laterthan simultaneously with the image signals of the radiation sensordevice.
 7. The apparatus of claim 1, wherein parameters of the read-outcircuit arrangement can be influenced by means of the dose signal. 8.The apparatus of claim 7, wherein parameters of the image signals of thesensors can be changed in dependence on the dose signal.
 9. Theapparatus of claim 1 wherein the totality of image signals of theradiation sensor device can be changed in dependence on the dose signal.10. The apparatus of claim 1, wherein the image signals can be appliedto the read-out circuit arrangement in dependence on whether a thresholdvalue of the dose signal is exceeded or not reached.
 11. The apparatusof claim 1, wherein in the radiation sensor device at least one meandose signal can be formed for at least two single sensors from the dosesignals generated by the dosimeters.
 12. The apparatus of claim 1,wherein in the read-out circuit arrangement the dose signal of a sensorcan be subtracted from the image signal associated with this sensor, theresultant difference signal being intended for further processing. 13.The apparatus of claim 1, wherein the radiation source is constructed asan X-ray tube and the radiation sensor device is constructed as an X-raydetector.
 14. An X-ray examination apparatus which includes an X-raytube, an X-ray detector for generating image signals, a read-out circuitarrangement which is associated with the X-ray detector in order toamplify/process image signals read out, at least one dosimeter which isarranged to measure a radiation dose, and an image processing device,characterized in that a dose signal generated by the dosimeter can beapplied to the read-out circuit arrangement and/or the image processingdevice.
 15. A method for processing X-ray images, in which method X-raysemitted by an X-ray tube are conducted to an X-ray detector which isarranged to generate image signals, comprises sensors and is associatedwith at least one dosimeter, the dosimeter generating at least one dosesignal which is indicative of the intensity of X-rays, said dose signalbeing applied to a read-out circuit arrangement no later thansimultaneously with the image signals in order to influence individualparameters of the image signals of the individual sensors and/or toinfluence the totality of image signals.
 16. A method of forming X-rayimages comprising the steps of: emitting X-rays from a source; detectingthe emitted X-rays; using at least a portion of the detected X-rays toform image information; gathering X-ray dosage information; and usingthe gathered dosage information with the image information to influenceindividual parameters of the image information prior to forming animage.
 17. The method of claim 16, wherein the step of using thegathered dosage information occurs substantially simultaneously with thestep of using the detected X-rays to form image information.